In general, carbon fibers are industrially produced by using rayon, PAN (polyacrylonitrile) and a pitch as raw materials. However, PAN is disadvantageous in that it is expensive and the carbonization yield is low. In this respect, pitches are cheap and thus economically attractive. However, low cost carbon fibers produced from isotropic pitches are poor in orientation and thus show low strength. Therefore, these pitches cannot provide a high performance fiber. By contrast, carbon fibers produced from optically anisotropic pitches which are called mesophase pitches have a highly oriented structure in which carbon crystallites preferentially align parallel to the fiber axis and thus have excellent mechanical characteristics, that is, high strength and a high modulus of elasticity. Extensive studies have been made to produce mesophase pitches as raw pitches for high performance carbon fibers from petroleum catalytic cracking residual oil, naphtha tar pitch, or coal tar pitch. However, a raw pitch thread obtained by melt-spinning of a pitch or an infusibilized fiber obtained merely by infusibilization of such a raw pitch thread is as extremely weak as 200 to 400 kg/cm.sup.2 in fiber strength and 0.5% in elongation and thus is subject to breaking due to bending, fretting, or contact against the surface of heating vessel thereof. In this respect, such a pitch fiber is rather different from synthetic fiber such as PAN. This weak fiber strength impedes the production of pitch carbon fibers. Various attempts have been made to try to eliminate these defects of pitch carbon fibers. Especially, many approaches have been done in vain for improving the strength of a raw pitch thread.
Furthermore, these starting materials such as coal tar, naphtha tar, and residual oil from fluid catalytic cracking of a petroleum fraction contain free carbon or other inorganic materials such as catalyst powder which will become ash content. These materials impede spinning of the pitch. Furthermore, raw pitch threads or carbon fibers containing these finely divided particles have defects which cause low strength. Various approaches have been proposed to remove these undesirable particles. However, these approaches are not sufficient to remove fine particles such as submicron particles. Thus, these still remain room for improvement of pitches.